How many multivalent elements are there? [closed]

According to this video there are 20 multivalent elements, and from the periodic table, I see 20, but according to multiple websites, including this, it says there are only 5. What am I not understanding? Are there 20 or 5? I want to know so I can memorize them. Also, are there any that aren't transition metals that are exceptions, i.e. non-metal multivalent? If so, may you please list them in your response?

Again talking about multivalent elements because I am learning ionic compounds, and these are part of it. Also, please keep high-level vocabulary to a minimum, I know some science vocabulary but i am only in grade 10.

Edit: I would like to add that i am referring to how many metals have roman numbers, i.e like Fe which has 2+ and 3+ charge both. are there more than 5 of these or are there 20?

To anyone with the same question: this article (thanks to Achem) and the article listed here article are very useful.

• You should not memorize all the periodic element's oxidation states randomly. Nobody will expect you to know all the elements. Learn the valency common elements which are mentioned in your grade-10 books. It is quite easy to see a pattern. Group I elements will exist as (+), group II elements (2+), etc. The transition elements show multiple valencies. Sep 26, 2022 at 1:02
• Both videos/websites are wrong. There are more than 20 elements which have multiple valency. All transition metals will show multivalency. All lanthanide and actinide elements show multivalency. Count how many transition elements are there in the periodic table. It is certainly more than 20. Sep 26, 2022 at 1:09
• This is exactly what I am trying to tell you, there are about 38 transition elements and they will show multivalency. So both websites are wrong. There are more than 20 multivalent elements. Fe(III) and $\ce{Fe^{3}}$ is the same thing. Sep 26, 2022 at 1:15